For a flash memory using a floating gate, which is the mainstream currently, there is such a problem that, along with the miniaturization of a memory cell, a threshold voltage (Vth) variation occurs caused by the interference due to capacitive coupling between floating gates of neighboring cells.
Accordingly, as a memory of a configuration appropriate to the miniaturization, the development of a resistance change type nonvolatile memory element having a layer in which resistance changes interposed between electrodes, is advanced. The resistance change type nonvolatile memory element is characterized in that the electric resistance of a resistive layer can be switched between two or more values by electric stimulus. The element is expected, because of the simplicity on the element structure and operation, as a nonvolatile memory element that allows the miniaturization and cost reduction.
Layers in which the resistance changes by an applied voltage include a layer of an oxide of an element selected from the group consisting of transition metals. As the oxides, there are nickel oxide (NiO), vanadium oxide (V2O5), zinc oxide (ZnO), niobium oxide (Nb2O5), titanium oxide (TiO2), tungsten oxide (WO3), titanium oxide (TiO2), cobalt oxide (CoO), tantalum oxide (Ta2O5) etc.
Although details of the operation principle of resistance change are not clear, a principle that, by application of a voltage to a resistance change layer, a current path referred to as a filament is formed in the resistance change layer and the resistance of the element changes according to the connection state between the filament and the upper and lower electrodes, and a principle that the resistance of the resistance change layer changes due to the movement of oxygen atoms at the boundary of the electrode and the resistance change layer, are reported.
By use of FIG. 16, an example of the operation principle of a resistance change type nonvolatile memory element (ReRAM: Resistive Random Access Memory) is described. A resistance change type nonvolatile memory element (a memory element) 610 being a general ReRAM has a parallel plate type stacked structure, in which a resistance change film (for example, a transition metal oxide film) 613 is interposed between a lower electrode 612 and an upper electrode 614 formed on an interlayer insulating film 611. A reference numeral 618 is a contact hole for connection with an external wiring. When a voltage is applied between the upper electrode 614 and the lower electrode 612, the electric resistance of the resistance change film 613 changes and can take two different resistance states (a reset state, a set state).
Regarding the operation mechanism of the resistance change type nonvolatile memory element 610, first, as an initial operation for allowing transition between two resistance states, a forming voltage is applied. The application of the forming voltage sets a state that a filament to be a current path may be formed in the resistance change film 613. After that, the application of an operation voltage (a set voltage or a reset voltage) changes the generation state of the filament to perform a set/reset operation, that is, writing or deletion.
In Patent Literature 1, a nonvolatile memory element is proposed in which an amorphous insulating layer containing a nickel oxide and a crystalline resistance change layer containing a nickel oxide are stacked between upper and lower electrodes, and it is described that insulation breakdown of the amorphous insulating film occurs and a stable filament is formed in the resistance change layer on a region through which a current flowed.
In Patent Literature 2, a nonvolatile memory element is proposed in which a hafnium oxide film having a composition of HfOx (0.9≦x≦1.6) and a hafnium oxide film having a composition of HfOy (1.8<y<2.0) are stacked between upper and lower electrodes and which has a high-speed and reversibly stable rewriting property.
In Non Patent Literature 1, a nonvolatile memory element is proposed in which Pt is used as upper and lower electrodes and the resistance change layer contains NiO, and it is described that a current path referred to as a filament is formed in a Ni oxide and the resistance changes. In addition, in Non Patent Literature 2, a nonvolatile memory element is proposed in which Pt is used as upper and lower electrodes and the resistance change layer contains TaOx, and it is described that the resistance changes by the movement of oxygen atoms at the boundary layer between the Pt electrode and TaOx.
In addition, a technology regarding a resistance change type nonvolatile memory element by use of a titanium nitride electrode as an electrode material for which etching processing is easy attracts attention. In Non Patent Literature 3, a nonvolatile memory element is proposed in which Pt is used as a lower electrode, HfOx or HfAlOx is used as a resistance change layer and an upper electrode contains TiN, and it is described that, by use of HfAlOx as a resistance change layer, the variation of operation voltage can be suppressed. In addition, in Non Patent Literature 4, it is described that it is possible to realize a resistance change operation by fabricating a stacked structure having TiN/TiOx/HfOx/TiN by oxygen annealing of a TiN/Ti/HfO2/TiN stacked structure.